An optical communication network is composed of optical fibers serving as media for propagating optical signals and optical transceivers for transmitting/receiving optical signals. A housing of the optical transceiver encloses an optical module for converting electric signals to optical signals and optical signals to electric signals and a printed board on which electronic elements and electric connectors, etc. are mounted for control.
The optical module has a package, in which optical elements which carry out photoelectric conversion such as a laser (light emitting element) and a photodiode (light receiving element) are mounted, and a flexible substrate. Generally, such optical modules are sometimes called, for example, ROSA in the light-receiving side and TOSA in the light-transmitting side.
A Can-type package or a Box-type package is used as the package in which the optical elements are mounted, and lead pins are used for input/output of signals. The lead pins penetrate through the flexible substrate and are fixedly attached thereto by soldering; alternatively, the lead pins and the flexible substrate are disposed so as to be approximately horizontal to each other, and the lead pins are fixedly attached with conductors on the flexible substrate by soldering.
Such configurations of the optical modules are often utilized in optical transceivers supporting transmission speeds of about several hundred Mbps to 10 Gbps. Recently, optical modules complying with the standard called XMD have been commercialized by several companies.
Incidentally, a width of about 200 microns is often used as the width of the lead pin of the optical module in order to maintain the mechanical strength thereof. This lead pin of the package and a microstrip line having a width of 100 microns cannot be fixedly attached to each other by soldering.
A flexible substrate of Japanese Patent Application Laid-Open Publication No. 2007-123741 (Patent Document 1) has a structure of a transmission path in which a microstrip line is changed to a coplanar line, and the coplanar line is connected with a lead pin. The width of the signal line of the coplanar line is widened more than the width of the signal line of the microstrip line, thereby facilitating the connection with the lead pin.
In the specification of the present application, a “microstrip line” refers to a transmission path in which a first main surface (hereinafter, referred to as “surface”) of a flexible substrate is provided with a signal line not sandwiched by surface ground lines, and a second main surface (hereinafter, referred to as “back surface”) of the flexible substrate is provided with a back-surface ground line overlapped with the signal line; a “coplanar line” refers to a transmission path in which the surface of the flexible substrate is provided with the signal line sandwiched by the surface ground lines with gaps therebetween, and the back surface of the flexible substrate is provided with the back-surface ground line which is not overlapped with the signal line; and a “grounded coplanar line”, which will be described later, refers to a transmission path in which the surface of the flexible substrate is provided with the signal line sandwiched by the surface ground lines, and the back surface of the flexible substrate is provided with the back-surface ground line overlapped with the signal line.
Above-described Patent Document 1 particularly employs a structure in which the distance between a signal line and each rectangular surface ground line is gradually narrowed since the line width of the signal line is gradually widened in a coplanar line in the region in which a microstrip line is changed to the coplanar line. In addition, this case employs a structure in which a back-surface ground line is branched so that the back-surface ground lines sandwich the signal line, wherein the distance between the ground line and the signal line is gradually increased.